Electric furnace for melting quartz



Aug. 29, 1961 K. VATTERODT 2,998,459

ELECTRIC FURNACE FOR MELTING QUARTZ Filed Sept. 11, 1959 2 Sheets-Sheet1 INVEN TOR BY KARL VATTERODT ATTOENEY Aug. 29, 1961 K. VATTERODT2,998,469

ELECTRIC FURNACE FOR MELTING QUARTZ Filed Sept. 11, 1959 2 Sheets-Sheet2 [E r 21 j 21 Fig a 2s 22 7 31 ii /-3o 27 Q. T30 7\429 LENA. E -1s EINVENTDR KARL VATTERODT BY W 7? ELECTRKI FURNACE FQR MELTING QUARTZ KarlVatterodt, Eerlin-Spandau, Germany, assignnr toPatent-TreuhandGesellschaft fiir Elelrtrisehe Gliihlampen rn.h.H.,Munich, Germany Filed 11, 1959, Ser. No. 839,339 Claims priority,application Germany tiept. 17, 1958 7 "Claims. ((2.13-22) This inventionrelates to furnaces for melting quartz or quartz-like glasses having ahigh content in silicic acid, and is of the type wherein the meltingcrucible is heated electrically.

In one of the well known constructions of such a furnace, heating Wiresare arranged in the form of a winding or of a grid on the inner side ofa fireproof jacket surrounding the cylindrical melting crucible, whichjacket is also enclosed by an outer water-cooled covering. The heatingwires consist of a refractory metal, as molybdenum or tungsten, aroundwhich a protective gas, as for instance hydrogen or a mixture ofhydrogen-nitrogen, is flowing during operation. Changes in length of theheating wires occurring in operation of the above-mentioned constructioneffects dislocations in the heating-wire arrangement whereby the wirescome into contact with one another or with the crucible. Nonuniformcurrent loads which occur thereby cause a premature destruction of theheated wires.

The above drawback was intended to be overcome by another well knownfurnace construction, viz. by means of arranging a greater number ofwires as uniformly as possible in parallel to the melting crucible axisso that the wires form a jacket surface of a coaxial cylinder. Theheating wires are fastened on both ends respectively to upper and lowerclamping rings. The upper clamping ring is connected firmly with thefurnace whereas the lower clamping ring is connected firmly with thelower furnace parts only, so that said lower clamping ring and lowerfurnace parts are held suspended by said heating wires. By virtue of theweight of the lower furnace parts a pull is exerted on the heatingwires, in consequence of which, as said heating wires elongate fromthermal expansion, said lower ring sinks correspondingly and maintainsthe wires always in taut condition. It was to be expected that by thismeans any dislocations caused by the thermal longitudinal extension andany touching of the wires with one another and with the melting cruciblewould be prevented.

It came, however, as a surprise that the heating wires are, duringoperation, exposed to different longitudinal extensions and that in thecourse of operation permanent extensions or shrinkings of differentextent are brought about. The shortest heating wires have to bear alonethe load of the lower furnace parts while the other heating wiresdevelop slack and become more or less curved. The extended shorter wiresbegin, therefore, to break after a proportionately short time whereasthe very much curved wires come into touch with one another or with thecrucible whereby they are also damaged. Life of such a heating deviceonly amounts to 100 to 350 operation hours. Thereupon, operation has tobe stopped and new parts have to be built into the furnace. Suchfrequent interruptions of the continuity of production requires greatexpense in material and time which has a bad effect on the price of thefinished object of quartz or of glass.

Consequently, it is an object of the present invention to disclose afurnace construction which does not show the above-mentioned drawbacksand guarantees a satisfactory continuous operation of the meltingfurnace for a time which amounts to a multiple of the usual life ofprior art furnaces. In order to explain the unexpected di fe entbehavior of the heating wires extensive examina Patented Aug. 29, 196itions were made with respect to the longitudinal extension of tungstenheating wires. It was found that the thermal expansion according tocertain well-known laws is not decisive therefor. In an axialsymmetrical arrangement in which like temperatures are at all places,similar heating wires must have similar thermal expansions. Theaforesaid experiments have shown that the observed differences are dueto recrystallization processes in the tungsten wires. Tungsten wiresare, as is known, manufactu'red from tungsten powder by subsequentpressing, sinten'ng, hammering and drawing. The finished tungsten wirehas a fiber structure which transforms again under strong heating. Thistransformation in structure, the recrystallization, takes place in anon-uniform manner, corresponding to the microscopic heterogeneous structure of the tungsten wire. Recrystallization has, therefore, quite adifferent efiect on each of the tungsten wires and brings about theobserved permanent different extensions or shrinkings of the individualheating wires.

At first, during heating up of the wires merely thermal expansion occursand that as a comparatively great variation in length which is alike toall heating wires. The transformation in structure in the tungsten wiresfrom the fiber structure of the new wire to the crystal structure doesnot ensue until strong heating prevails. During continuous operation ofthe furnace the heating wires are subjected to but trifling changes intemperature. The variations in length resulting therefrom and fromrecrystallization are small in comparison to the variation in lengthduring heating up. But the comparatively small variations in length leadalso to a rapid destruction of the heating wires as was apparent fromthe aforementioned furnace structure wherein the heating wires are fixedbetween upper and lower clamping rings.

The present invention proceeds from these observations and arrives attwo types of furnaces for melting quartz or quartz-like glasses having ahigh content of silicic acid with a cylindrical melting cruciblesurrounded by electrically heated wires arranged in parallel to the axisof the crucible and being under tensile stress, these heating wiresbeing rigidly connected at one end to the end parts of the furnacewhilst the other end of each heating wire, together with a guide memberor stud mounted thereon, is individually led through a bore therefor inan annular end part of the furnace, wherein each heating wire is movablysupported in order to compensate for its variation in length. In bothfurnace constructions of the present invention two systems of differentstrength are provided for every single heating wire, exerting tensilestress and the change from the greater tensile stress of the strongersystem to the smaller tensile stress of the Weaker system is effectedautomatically.

For the greater tensile stress in the first type of construction theweight of the lower furnace parts suspended on the heating wires servesfor neutralizing the variations in length common to all heating wiresand mainly occurring during heating up. For the smaller tensile stressthere is mounted in each bore a compression spring surrounding theheating wire with its guide member and being arranged between asupporting area in the bore and a guide member end section or head oflarger diameter for neutralizing the differences of extension of lengthof the individual heating wires occurring during operation.

In the second type of furnace construction the two systems of differentstrength exerting tensile stress and being provided for each heatingwire, are in the form of two compression springs of different strengthssurrounding the heating wire with its guide member in the bore. They arearranged in such a manner that the weaker spring between one guidemember end section of larger diameter and a disc being axiallydisplaceable on the guide member cannot be released until the strongerspring between the disk and a supporting area in the bore correspondingto the extension in length of a heating wire during heating up, isreleased to such an extent that the disc bears against a fixed stopwithin the bore, thereby preventing a further relaxation of the strongspring.

This invention guarantees that all heating wires, independent from moreor less variations in length of each single wire, are in expandedcondition in every moment of heating up and of operation whereby theyheat the crucible in uniform manner. The invention also guarantees thatthere does not occur any non-uniform current or tensile stressoverloadings which may cause a too early destruction of some or all ofthe heating wires.

The accompanying drawings show two embodiments of the invention and likenumerals of reference indicate similar parts throughout the severalviews.

FIG. 1 shows a side View, partly in section, of a melting furnace forthe manufacture of quartz tubing according to one of the embodiments ofthe invention.

FIG. 2 shows a cut-out from FIG. 1 in larger scale, viz. the holdingdevice of the upper end of the heating wire.

IS. 3 shows, similar to FIG. 1, -a side view, partly in section, of amelting furnace for the manufacture of quartz tubing according toanother embodiment of the invention, in which a large part of theheating zone is omitted.

FIG. 4 shows a cut-out from FIG. 3 in enlarged scale, viz. the holdingdevice of the upper end of a heating wire in cold condition.

H6. 5 shows the same heating wire end as FIG. 4, as in operation underheated condition.

The melting furnaces for the manufacture of quartz tubing shown inFTGURES 1 and 3 are operated in connection with a drawing machine whichis not here shown. Such drawing machine is arranged below the furnaceand continuously draws the quartz tubing which leaves the furnace,whereby the tubing is formed by appropriate tapering down to desireddiameter and wall thickness. The furnaces shown in the drawing have aninput of 60 kw. and are operated on a low-voltage heavy currenttransformer.

A hollow melting crucible 1, open at its upper end, is shown in E6. 1,into which raw material, namely, rock crystal for example, is fed. Thebottom of the crucible l is formed by an annular drawing nozzle 2 with acoaxial mandrel 3. The raw material is melted in the crucible and flowsfrom said nozzle and accordingly emerges therefrom formed in tubularshape in plastic condition and passes immediately to the aforementioneddrawing machine. The mandrel is hollow and connected therewith is a pipe4- for feeding an inert gas, or reducing gas, such as hydrogen andnitrogen respectively or a mixture thereof, to the interior of thequartz tubing as it is formed, thereby prevening surrounding atmospherefrom penetrating into the nozzle through the quartz tubing.

As shown in this embodiment, the crucible 1 is surrounded with heatingwires 5 arranged in parallel to the crucible axis and spaced at equaldistances from each other, forming a coaxial cylindrical cage or jacket.The heating wires 5 are composed of tungsten, and during operation, justas with the inner surface of the mandrel, are scavenged with a reducinggas of which a mixture of hydrogen and nitrogen is again an example. Asin operation these heating wires carry a current of about 150 amps.each, the diameter thereof may well be 2.8 mm.

The outer lateral closure of the heated zone of the furnace is formed bya metallic water-cooled furnace jacket 8. A cylindrical muflle 6 ofzirconium oxide is arranged outside of the crucible and inside the waterjacket in spaced coaxial relation to both. The cage of heating wires 5is enclosed by the mufile. Under the bottom ends of the muflle 6 andfurnace jacket 8 there is a bottom closure ring 8a the central opening.of which is large enough to permit the heating wires to passtherethrough to depend therebelow. In the space above said closure ringand between said muffle and furnace jacket is a pressed-in layer 7 ofzirconium oxide powder for heatinsulating purposes.

At the lower ends of the heating wires 5 is a horizontally disposedclamping ring 9 to which all of said wires are securely attached. Belowsaid clamping ring is a cover cap it) and above said clamping ring is acooling plate 11 with a central opening similar to that of the closurering 8:: admitting free passage of the heating wires therethrough. Thecover cap 10 and lower cooling plate ii are secured to and supported bysaid clamping ring 9 and together constitute the lower furnace end whichis suspended by the heating wires for its entire support. The distancebetween the crucible and said lower furnace end increases withlongitudinal extension of the wires 5 due to thermal expansion, as aresult of which there will be variable spacing between the closure ring8a and the proximate cooling plate 11. In order to prevent theprotective gas scavenging the heating wires 5 from escape out of thisintermediate space, there is provided therearound an oil immersion sealconstituted by annular trough 12 at the periphery of the cooling plate11, into which depends a cylindrical apron 12a secured to the bottom ofthe Water jacket 8. If preferred, the seal may be eifected by means of aflexible folding surrounding sheath or bellows 13 of copper or othersuitable material may be employed to supplement or in place of the oilseal.

At the top of the muffie 6 and jacket 8 is an upper transverse coolingplate 14, which is electrically insulated from the metal of the waterjacket by the interposition of an insulation gasket 14a therebetween. Inaddition to the transverse cooling plate 14, there is superposed thereonand around the crucible, a cooling ring 16 which is secured to saidplate and is electrically conductive. Both the said cooling plate andcooling ring are appropriately constructed for water cooling thereof. Italso is to be noted that the transverse cooling plate 14 has a centralopening through which the crucible and the cage of heating wires maydepend from the superposed cooling ring.

Mounting of the heating wires for suspension from said cooling ring 16,is shown with respect to one of said Wires in FIG. 2, and description ofthe one will suifice for all. On the upper end of the tungsten heatingwire 5, secured thereon in any suitable manner, as by sweating orpinching, is a cylindrical guide member or floating stud 17 slidablylocated in a socket therefor which is constituted as a hole ofrelatively large diameter at its upper portion 18, large enough tofreely receive the stud head 21, and a relatively smaller diameter lowerportion 19 for receiving and guiding the shank of said stud 17. Betweenthe larger and smaller hole portions is a transverse shoulder orsupporting area 2% for the lower end of a helical spring 23 the upperend of which bears against the under stop surface 22 of said stud head21. The surface areas of the smaller diameter hole portion 19 and thestud surface proximate thereto are considerably greater than acorresponding length of heating wire and therefore sufficientlyextensive to lead off considerable heat transferred thereto by therespective heating wire and consequently will keep the headed end of thestud at a temperature not over 200 C. The spring 23 is of appropriatestrength so as to be substantially fully compressed by the weight of thelower furnace end hereinabove identified as comprising clamping ring 9,cover cap it and cooling plate 11.

At the top end of each guide member or stud 17 is attached a strandedflexible lead connection 25 the other end of which is attached to theupper cooling ring 16, there of course being as many of said flexiblelead connections as there are heating wires. As all of the bottom endsof the heating wires 5 are secured to clamping ring 9, it will bereadily appreciated that by applying a source of voltage to the uppercooling ring 16 and to the bottom clamping ring 9 current willfiow inparallel through all of said heating wires 5. For illustrative purposesthe output from a transformer T is shown having wired connections withthe top cooling plate 14 and to the fixed bottom plate 15 of coolingjacket 8, it being remembered that the cooling jacket and the top plateare electrically separated by insulating gasket 14a. A flexible cable 24to carry the current across the oil seal to the lower suspended furnaceend is provided. When current is applied and the wires heated thereby,they all expand more or less in their longitudinal direction. Thealteration in length common to all of the heating wires is compensatedfor by gravitational downward pull of the lower furnace parts. Therewill be some of the wires, however, that elongate more than the saidcommon alteration or lengthwise expansion, and this difference isbalanced by and the wire kept taut by upward pull exerted by theparticular spring 23 associated with that particular wire. Owing to thefact that the difference in elongation is small compared to thealteration in length common to all of the wires, the tensile stress ofthe heating wires having greater elongation only decreases a little, sothat the heating wires which remained shorter are not overloaded inconsequence of the tensile force of their somewhat more compressedsprings. Thus constructed, furnaces have proven to be highly successfulwith life duration up to 2000 operating hours.

Referring now to the embodiment of the invention illustrated in FIG. 3,observation is made at the outset that whereas in FIG. 1 the lowerfurnace parts are movable and suspended on the heating wires, thismodification shows the lower furnace parts fixed with respect to themain body of the furnace. The required tensile stress and thecompensation for the differential in elongation is here supplied bysprings only.

Shape and arrangement of the melting crucible 1, the heating wires 5,the muffle 6, the insulating layer 7 and the cooling jacket 8, are likethose shown in FIG. 1. The heating wires 5 are clamped firmly by awater-cooled lower contact ring 26 which is connected in a rigid mannorwith the water-cooled closing cap 10 as well as with the rest of thefurnace. On the furnace there is located in electrically insulatedmanner the upper contact ring 27 which is provided with sockets orborings for the several heating wires 5.

Because of the fact that the tungsten wires 5 while initially heating uphave a considerable longitudinal thermal expansion, but during operationsome may elongate beyond the comrnon-to-all initial expansion a furthersmall amount, there are provided for each heating wire 5 two springs ofdifferent strength. The stronger spring for each wire balances thelongitudinal extension thereof during the heating-up period and iswithout any greater effect thereafter during operation. Only the weakorspring has an effect on the heating wire 5 in operation for elongationoccurring beyond the extension common to all of the heating wires.

As is shown in 'FIGS. 4 and 5, more clearly than in FIG. 3 by use oflarger scale, each socket or boring consists, just as in FIGS. 1 and 2,of a main section 18 of larger diameter than and adjacent a smallersection 19 coaxially therebelow. Into the upper end of the main section18 of said socket there fits a shell 28 of such a wall thickness thatthe end section or head 21 of the stud or guide member 17 may beinserted into shell 28 with sufficient play for axial movement. The mainsection 18 of the socket contains what is herein termed a strongerspring 29 which rests on the supporting area 20. Above said spring andin said socket there is a sliding disc 30 which may be shifted axiallyupwardly to the shell lower rim. Above and seating against said disc 30is what is herein termed a weaker spring 31 located in said shell 28 andalbuts at its upper end against the stop shoulder 22 and on the underside of head or end section 21 of the stud or guide member 17. The sizesof surfaces of the adjacent section 19 of the socket or boring and ofthe lower section of the guide member or stud 17 sliding within theaforesaid section 19, are chosen in such a manner that a suflicientdeduction takes place by the upper contact ring 27 of heat transferredduring operation out of the interior of the furnace by heating wire 5through heat conduction that the temperature in end section or head 21of the stud 17 does not rise above 200 C.

As shown in FIG. 4, both of the springs 29 and 31 are completely pressedtogether, which is the condition before the wires are heated. Betweenthe disc 30 and the lower rim of shell 28 there exists, in thatcondition, a distance a which corresponds nearly to the longitudinalextension of the heating wire 5, common to all of the wires, when heatedup. During the heating up period of the wire its thermal expansion iscompensated for and the wire kept taut by the stronger spring 29 whichpushes upward until the disc 30 abuts from below against the shell 28 asshown in FIG. 5. After this proportionately considerable extension hastaken place only smaller differences in extension occur in severalheating wires 5 which the weaker spring 31 then compensates for, takingup the slack and keeping the wire taut. The several weaker springs 31therefore take care of the slightly different variations of wire lengthsand have adequate tensile force which will suffice to keep any one ormore of the wires taut that may have, during operation, an expansiongreater than that which is common to all of the wires.

It will of course be understood that a coaxial mandrel such as thatshown in FIG. 1 is also used in the furnace of FIG. 3 and in conjunctiontherewith an appropriate gas such as the hydrogen-nitrogen mixture isalso used. A like gas is also present around the heating wires. Mentionmay be made of the fact that since in FIG. 3 the lower furnace end isfixed to the water jacket, there will be direct electrical connectionthereby and thus current from the transformer, example of which is shownin FIG. 1, will be conducted to the lower end of the heating wires. T oguarantee good electrical contact between the upper contact ring 27 andeach heating wire 5, there is provided for each single heating wire,just as in FIGS. 1 and 2, a flexible current in-lead 25.

With this furnace construction of FIGS. 35, there was obtained also alife duration of about 2000 operating hours.

I claim:

1. An electric furnace of the character described, com prising upper andlower end parts for the furnace, a cooling jacket between said endsparts, a crucible longitudinal of and within said cooling jacket, anannular series of heating wires parallel to said crucible and betweensaid crucible and cooling jacket, said wires being subject toelongation, by thermal expansion when heating up and during operation,each said wire having two systems of different strength applied theretoexerting tensile stress longitudinally thereof, the stronger systembeing effective to maintain tension in the heating wire during heatingup thereof and the weaker system being effective for maintaining tensionin the heating wire in event of greater elongation during operation, andmeans for compensating for difierences in elongation of said heatingwires from each other, said means effecting an automatic change from thegreater tensile stress of the stronger system effective on said wires tothe smaller tensile stress of the weaker system effective on said wires.

2. An electric furnace in accordance with claim 1, wherein said strongersystem comprises gravitational suspension of the said lower end parts ofthe furnace at the lower ends of said heating wires and the weakersystem comprises individual springs for each said wire for neutralizingthe differences of extension in length of the respective heating wire.

3. An electric furnace in accordance with claim 1, wherein said upperend part of the furnace provides a plurality of sockets, one for eachheating wire, and wherein a stud is slidable in said socket for eachwire and secured to said wire, and a spring in each said socket inengagement with said stud for sliding the same and tension said wire.

4. An electric furnace in accordance with claim 1, wherein two springsof different strength are provided in each socket for constituting saidtwo systems exerting tensile stress on the rmpective heating wires.

5. An electric furnace in accordance with claim 3, wherein two springsare provided in each socket constituting said two systems, and wherein adisc is interposed between said springs and a fixed shell provided abovethe lower one of said springs and above said disc and receiving theupper one of said springs therein.

6. An electric furnace in accordance with claim 3, wherein said socketand stud have lower end portions proximate one to the other for heattransfer to thereby 1 after which the upper and weaker spring may expandupon further thermal elongation of said wire.

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